1. Field of the Invention
The patent invention relates to a molded plastic package for a semiconductor device that includes a metal heat spreader having an exposed surface. The heat spreader has a raised seal ring around the periphery of that exposed surface that prevents plastic molding compound from bleeding over the exposed surface of the heat spreader during the encapsulation process.
2. Brief Description of the Art
Molded plastic electronic packages provide environmental protection to integrated circuit devices. Packages such as the QFP (quad flat package) and PLCC (plastic-leaded chip carrier) protect an encapsulated device from contaminants such as moisture as well as from mechanical shock. One molded plastic package is illustrated in U.S. Pat. No. 4,707,724 to Suzuki et al. The package has a leadframe with a centrally positioned die attach pad. The semiconductor device is bonded to the pad and electrically interconnected to the inner ends of the leadframe. A polymer molding resin encapsulates the device, die attach pad and inner lead ends.
One disadvantage with molded plastic packages is poor thermal dissipation. During operation, the semiconductor device generates heat which must be removed to maintain the operating integrity of the device. While some heat is dissipated through the bonding wires and leadframe, the remainder is absorbed into the molding resin. The molding resin is a poor thermal conductor so the device temperature will increase unless the power provided to the device is limited.
Incorporating a heat spreader into the molded plastic package provides an enhanced path for thermal dissipation. As a result, more power may be provided to the semiconductor device without a resultant excessive increase in device temperature. The heat spreader, which is usually copper or aluminum, is embedded in the molding resin, usually below the die attach pad, reducing the amount of molding resin through which heat must pass to reach a surface of the package.
In addition to copper or aluminum, heat spreaders formed from composites having a copper or aluminum component and a lower coefficient of thermal expansion component are disclosed in U.S. Pat. No. 5,015,803 to Mahulikar et al.
Various other improvements to heat spreaders have been made. For example, U.S. Pat. No. 5,367,196 to Mahulikar et al. teaches the use of an adhesion enhancing layer on an aluminum heat spreader. This adhesion enhancing layer may be anodized aluminum, chromium, zinc, a mixture of chromium and zinc or a mixture of chromium and phosphorous. This layer improves both thermal dissipation and adhesion to the molding resin.
U.S. Pat. No. 5,530,295 to Mehr discloses a nickel-plated copper or anodized aluminum heat spreader that has a pair of oblique steps that engage the dielectric housing of the electronic package and ensure the heat spreader does not become detached from the package. The heat spreader in this patent also contains a pair of alignment holes that are adapted to receive corresponding mold alignment pins.
U.S. Pat. No. 5,608,267 to Mahulikar et al. discloses a heat spreader made of aluminum, titanium, zinc, magnesium and alloys thereof and having an anodization layer coating thereon.
U.S. Pat. No. 5,641,987 to Lee discloses a heat spreader for a semiconductor package containing a plurality of rectangular openings broadening radically outwardly from a center pad. These openings are surrounded by regularly spaced indentations and alternating outwardly pointing and downwardly pointing projections.
U.S. Pat. No. 5,652,461 to Ootsuki et al. teaches the use of a convex heat spreader for a semiconductor device. The heat spreader is made from a high heat-conductive material such as copper, aluminum, silver, gold and alloys thereof.
U.S. Pat. No. 5,656,864 to Mitsue et al. discloses a semiconductor device having a two-part molded package and a supporting member made of anodized aluminum alloy plate that has through holes therein so that a molding resin flows during the molding of the package.
U.S. Pat. No. 5,814,878 to Hirakawa et al. is directed to heat-sink that has grooves formed in its internal top surface to suppress thermal resistance and to improve loss heating radiating efficiency.
U.S. Pat. No. 5,757,070 to Fritz teaches an integrated circuit package in which the lead fingers are spaced away from a copper heat spreader (or heat sink).
U.S. Pat. No. 5,793,613 to Poinelli et al. is directed to a molded plastic material package for a semiconductor electronic device that has a fully insulated inner heat sink material.
U.S. Pat. No. 5,859,477 to Fehr teaches a fully insulated heat-sink that has a side surface formed with a relief to allow flowable mold resin to pass thereby.
All of the above-noted U.S. Patents are incorporated herein by reference in their entireties.
One of the difficulties encountered during the encapsulation of molded plastic packages with a drop-in exposed heat spreader is the flashing or bleeding of the mold compound onto the exposed area of the heat spreader. This resin bleed problem is influenced by several factors; such as proper mold gating design, flow characteristics of the mold compound, molding parameters, thickness of the heat spreader, and flatness of the exposed area of the heat spreader. This latter factor of flatness is often difficult to achieve due to several inherent characteristics of heat spreader manufacturing processes, namely, the presence of burrs (unwanted metal protrusions left on the work piece as a result of mechanical operations) along the edges of the exposed area, surface imperfections, and/or foreign materials left on the exposed area of the heat spreader.
The presence of such an undesirable thin layer of plastic molding compound on the exposed area acts as an insulation to the heat spreader which could adversely affect the thermal performance of the semiconductor device. This resin bleed is also cosmetically unacceptable to a number of customers. Furthermore, this thin layer of resin sitting on top of the exposed heat spreader area has the potential of breaking off as loose particles which could create mechanical or electrical contact problems to electrical test handling systems. The present invention offers a solution to these problems caused by this thin layer of plastic molding compound on the heat spreader's exposed surfaces.